1. Field of the Invention
The present invention relates to a power supply failure early warning detector and, more particularly, to a circuit for detecting, in advance, by a desired length of time, the imminent failure of electric output power provided by an electric power supply due to the reduction or loss of input power.
2. Description of the Prior Art
There are many instances where it is necessary or desirable to provide an early warning of the imminent occurrence of an undesirable event. One such instance is in the case of an electric power supply which provides electric output power for a critical supply load where it is necessary to detect the imminent failure of the electric output power. Such critical supply loads include computers, medical equipment, communications equipment, military equipment and the like. In many cases, loss of electrical power, without warning, may result in disastrous consequences. For example, computers include memories which require power for the maintenance of the memory contents. If electric power is lost, without warning, all the data stored in the memory would typically be lost. In this case, as in many others, an early warning signal of imminent failure of electric power enables the powered system to commence orderly shutdown actions, such as the transfer of data to another storage device, thus preventing the loss of data or other undesirable event.
In view of this requirement, many power supplies include an early warning power failure detection circuit. Typically, such circuit includes a comparator which compares the instantaneous voltage upon an energy storage reservoir capacitor to a reference voltage which is arbitrarily chosen to be a predetermined amount greater than the minimum capacitor voltage at which normal power supply operation is possible. When the instantaneous voltage reaches this reference voltage, imminent power supply failure is signalled.
Such a scheme is undesirable because it fails to take into account the energy storage reservoir capacitor's characteristics which are a function of the power supply and the load current. The greater the load current, the more rapidly the energy storage reservoir capacitor will discharge. Knowing this, the reference voltage is selected based upon the worst case conditions, which is under maximum load conditions. Under partial load conditions, imminent power failure is signalled long before it is necessary to do so. The result is a high false alarm rate and the circuit's inability to respond in those situations where input power is lost only temporarily and is subsequently restored. Under such circumstances or under brown-out conditions, it is often unnecessary to signal imminent power failure, but this invariably occurs.
Another commonly used scheme is to monitor the alternating current input to a power supply. Each peak of an AC signal is used to trip a monostable multivibrator having a time delay of approximately 17 milliseconds, which is just longer than the time interval between consecutive peaks of an AC signal at 60 Hz. When the monostable multivibrator times out without sensing a new peak in the AC input signal, loss of AC power is assumed and imminent failure of the electric power supply is indicated.
This technique, too, has its limitations. The circuit may trigger in the presence of a decreased AC input voltage even though there is sufficient voltage to maintain operation of the power supply. Furthermore, the equipment operated by the power supply may in fact be able to maintain operation through many cycles of loss of AC input power such that if power comes back on after only a few cycles, it is not necessary to shut down. Since loss of a cycle or two in AC sources is common, these circuits too provide unnecessary false alarms.
Numerous other examples exist where it would be desirable to provide an early warning in the presence of a rising or lowering physical parameter. Another example would be in the case of a temperature sensing device where it would be desirable to provide an early warning of an imminent and undesirable increase or decrease in a temperature. Often these circumstances, too, are complicated by changing rates of the physical parameter. A circuit satisfying these requirements has been unavailable heretofore.